This invention relates to improvements in electric power systems for aircraft and, more particularly, to such systems which provide both starter and generator functions. The present invention has particular applicability to starter generators for helicopters.
Typically, in airborne electrical power generation systems, it is desirable to have a single system which provides both the starter and generator functions. The weight savings on an aircraft can be substantial when a dedicated starter is eliminated. For this reason, electrical power generating systems which are capable of providing engine start functions can provide both cost and weight savings.
The starter-generator commonly utilizes a machine that combines an induction motor with a synchronous generator. In this machine, a rotor-shaped stator is positioned inside a squirrel-cage induction rotor which has an array of magnets attached on the outer diameter thereof. The compound/dual machine operates as a starter by using the induction rotor to accelerate the permanent magnet rotor up to some low synchronous speed, where ac power can be applied to the outside stator of the synchronous-generator so as to lock-in the permanent magnet rotor synchronously with the rotating field created in the armature (stator) of the synchronous-generator.
Frequently, the weakest link in the design of a starter-generator is the brushes. The carbon brushes utilized in starter-generators wear rapidly when brush temperatures exceed 450 degrees F. During startup, the starter generator brush temperatures can exceed 800 to 1000 degrees F. or higher, thus significantly exceeding the 450 degree F. threshold for rapid wear. The life of the starter-generator is frequently governed by the life of the brushes. With the development of higher horsepower turbine engines the starter-generators utilized to start such engines have been experiencing rapid brush and armature commutator wear. Besides the cost of overhauling the starter generator after relatively short hours of use the rapid wearing of the brushes causes a build up of carbon dust inside the starter generator, in the engine compartment and outside on the cowling as well.